I run 77gr and 75gr bullets in my 18" 1:7 twist SPR rifle. However, both of my 16" 1:7 M4 clone carbines seem to group better with 60gr, 62gr, 68gr, and 69gr bullets. One of my rifles is a standard chrome lined 1:7 twist M4 contour barrel, and it shoots the 60gr, 62gr (SS109), and the 69gr...